Raman Spectra of Poly(p-phenylenevinylene)s with Fluorinated Vinylene Units: Evidence of Inter-ring Distortion

The Raman spectrum of poly{2-methoxy-5-[(2-ethylhexyl)oxy]-
1,4-phenylenedifluorovinylene} (MEH-PPDFV) is reported and
compared with that of a well-known non-fluorinated reference
polymer, namely poly{2-methoxy-5-[(2-ethylhexyl)oxy]-1,4-phenylenevinylene}
(MEH-PPV). The Raman spectra of both polymers
are assigned on the basis of density functional theory calculations
of the corresponding oligomers. The main differences
between vinylene fluorinated and non-fluorinated structures
deal with the intensity, frequency shift and broadening of CC
vinylene stretching. Additional differences concern the relative
intensities of CC phenylene and vinylene stretching as well as
the deformation modes in the range 1250-1350 cm1. It is
shown that these effects are due to the larger distortion from
planarity of the fluorinated polymer, compared with the nonfluorinated
counterpart, induced by repulsive interactions between
the fluorine atoms on the vinylene units and the
oxygen atoms of the alkoxy groups on the aromatic rings.